[gdb/testsuite] Fix timeout in gdb.tui/main.exp

[gdb/testsuite] Fix timeout in gdb.tui/main.exp

With test-case gdb.tui/main.exp we run into:
...
WARNING: timeout in accept_gdb_output
PASS: gdb.tui/main.exp: show main after file
...

The problem is that this command:
...
Term::command "file [standard_output_file $testfile]"
...
tries to match "(gdb) $cmd", but due to the long file name, $cmd is split up
over two lines:
...
   16 (gdb) file /data/vries/gdb/leap-15-4/build/gdb/testsuite/outputs/gdb.tui/main/ma
   17 in
   18 Reading symbols from /data/vries/gdb/leap-15-4/build/gdb/testsuite/outputs/gdb.t
   19 ui/main/main...
   20 (gdb)
...

Fix this by matching "Reading symbols from" instead.

Tested on x86_64-linux.